What is difference between deep sleep and REM sleep?

Deep sleep and REM sleep are different stages of sleep that serve distinct functions and exhibit characteristic brain activity.

Deep sleep, or slow-wave sleep, is when the brain produces large, slow brain waves. This stage is considered very restorative, and it's hard to wake someone from this state. When awakened from deep sleep, people are often confused and can easily return to sleep 1. During this stage, the body experiences decreased heart rate, breathing, and muscle activity, and it's the time when the body conducts tissue repair, growth hormone release, and overall physical renewal.

REM sleep, or rapid eye movement sleep, is characterized by rapid movement of the eyes, vivid dreaming, and increased brain activity that resembles an awake state. However, during REM sleep, serotonin and norepinephrine levels drop significantly, and the body is effectively paralyzed to prevent acting out dreams. REM sleep is thought to support brain functions like learning, memory consolidation, and emotional processing 1 2.

Over the course of a night, sleep cycles between deep sleep and REM sleep, with the proportion of REM sleep increasing in the later cycles closer to morning. If someone's deep sleep is disrupted and they begin sleeping at a time when REM would normally dominate, their sleep will consist mostly of REM, and they will be significantly deficient in deep sleep. Thus, both stages are reciprocally regulated and are essential for different aspects of restoration and function 3.

In summary, both deep sleep and REM sleep are critical to overall health, playing distinct and important roles in the body's physical and mental restoration processes.